Hearing aids for people with hearing difficulties have been known as speech information transmitting devices. Hearing aids can be divided into air conduction hearing aids that transmit sound vibrations to the cerebral auditory areas via the eardrum, and bone conduction hearing aids that transmit sound vibrations directly to the human body, for example to the skull, not via the eardrum. Such hearing aids are used by attaching an earphone or vibrator to a part of the human body. Recently, structures have become known which enable the transmission of speech information by transferring supersonic vibrations to the cerebral auditory areas through a vibrator. For example, Japanese Unexamined Patent Publication No. 2001-320799 discloses a structure in which a sound signal is subjected to DSB (double sideband) amplitude modulation and transmitted to the human body through an ultrasound transducer.
However, hitherto known speech information transmitting devices still have room for improvement with respect to increasing the discriminability of speech.